This patch achieves the functionality of reordering .text.hot sections before
.text section.

In lld, the sections are ordered by an integer Rank. The Rank is calculated
in getSectionRank(), which returns an integer depending on the input Section
type. lld, instead of using simple numbers (0, 1, 2, ...), it uses a series
of flags by setting bits of the returning Rank. The smaller a Rank is, the
section will be put more towards the front.

In order to put hot symbols in front of text section, we need lld is used
with -z keep-text-section-prefix. With the flag set, the section prefix (.hot)
will be preserved when the function getSectionRank() is called. In the function,
we check if the section has a .hot prefix. If so, we need to set 'RF_EXEC_HOT'
flag to the return Rank. Otherwise, we set 'RF_EXEC' (which is normal .text
section). We set 'RF_EXEC_HOT' to be 1<<13 and 'RF_EXEC' to be 1<<14, so sections
with '.hot' prefix is smaller in Rank than other text sections.

In the test, we reorder the .text.hot and .text in the orignal text-section-prefix.s,
which should test the patch.

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index b6a70a46327..d4b3e216860 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -758,16 +758,17 @@ static bool isRelroSection(const OutputSection *Sec) {
 // * It is easy to check if a give branch was taken.
 // * It is easy two see how similar two ranks are (see getRankProximity).
 enum RankFlags {
-  RF_NOT_ADDR_SET = 1 << 27,
-  RF_NOT_ALLOC = 1 << 26,
-  RF_PARTITION = 1 << 18, // Partition number (8 bits)
-  RF_NOT_PART_EHDR = 1 << 17,
-  RF_NOT_PART_PHDR = 1 << 16,
-  RF_NOT_INTERP = 1 << 15,
-  RF_NOT_NOTE = 1 << 14,
-  RF_WRITE = 1 << 13,
-  RF_EXEC_WRITE = 1 << 12,
-  RF_EXEC = 1 << 11,
+  RF_NOT_ADDR_SET = 1 << 28,
+  RF_NOT_ALLOC = 1 << 27,
+  RF_PARTITION = 1 << 19, // Partition number (8 bits)
+  RF_NOT_PART_EHDR = 1 << 18,
+  RF_NOT_PART_PHDR = 1 << 17,
+  RF_NOT_INTERP = 1 << 16,
+  RF_NOT_NOTE = 1 << 15,
+  RF_WRITE = 1 << 14,
+  RF_EXEC_WRITE = 1 << 13,
+  RF_EXEC = 1 << 12,
+  RF_EXEC_HOT = 1 << 11,
   RF_RODATA = 1 << 10,
   RF_NOT_RELRO = 1 << 9,
   RF_NOT_TLS = 1 << 8,
@@ -823,8 +824,12 @@ static unsigned getSectionRank(const OutputSection *Sec) {
   if (isExec) {
     if (isWrite)
       rank |= RF_EXEC_WRITE;
-    else
-      rank |= RF_EXEC;
+    else {
+      if (isSectionPrefix(".text.hot.", sec->name))
+        rank |= RF_EXEC_HOT;
+      else
+        rank |= RF_EXEC;
+    }
   } else if (isWrite) {
     rank |= RF_WRITE;
   } else if (sec->type == SHT_PROGBITS) {
diff --git a/lld/test/ELF/text-section-prefix.s b/lld/test/ELF/text-section-prefix.s
index e39536da387..a0c77652511 100644
--- a/lld/test/ELF/text-section-prefix.s
+++ b/lld/test/ELF/text-section-prefix.s
@@ -7,8 +7,8 @@
 # RUN: ld.lld -z nokeep-text-section-prefix %t -o %t4
 # RUN: llvm-readelf -l %t4 | FileCheck --check-prefix=CHECKNO %s
 
-# CHECK: .text
 # CHECK: .text.hot
+# CHECK: .text
 # CHECK: .text.startup
 # CHECK: .text.exit
 # CHECK: .text.unlikely