Detect LLVM 7 without specifying binaries explicitly.

Before this commit LLVM 7 was supported, but only if one explicitly
provided LLVM_CONFIG= and CLANG= paths.  As LLVM 7 is the first
version that includes our upstreamed debugging and profiling features,
and as debian is planning to default to 7 due to wider architecture
support, it seems good to support auto-detecting that version.

Author: Christoph Berg
Discussion: https://postgr.es/m/20180912124517.GD24584@msg.df7cb.de
Backpatch: 11, where LLVM was introduced

config/llvm.m4

@@ -13,7 +13,7 @@ AC_DEFUN([PGAC_LLVM_SUPPORT],
   AC_REQUIRE([AC_PROG_AWK])
 
   AC_ARG_VAR(LLVM_CONFIG, [path to llvm-config command])
-  PGAC_PATH_PROGS(LLVM_CONFIG, llvm-config llvm-config-6.0 llvm-config-5.0 llvm-config-4.0 llvm-config-3.9)
+  PGAC_PATH_PROGS(LLVM_CONFIG, llvm-config llvm-config-7 llvm-config-6.0 llvm-config-5.0 llvm-config-4.0 llvm-config-3.9)
 
   # no point continuing if llvm wasn't found
   if test -z "$LLVM_CONFIG"; then
@@ -31,7 +31,7 @@ AC_DEFUN([PGAC_LLVM_SUPPORT],
 
   # need clang to create some bitcode files
   AC_ARG_VAR(CLANG, [path to clang compiler to generate bitcode])
-  PGAC_PATH_PROGS(CLANG, clang clang-6.0 clang-5.0 clang-4.0 clang-3.9)
+  PGAC_PATH_PROGS(CLANG, clang clang-7 clang-6.0 clang-5.0 clang-4.0 clang-3.9)
   if test -z "$CLANG"; then
     AC_MSG_ERROR([clang not found, but required when compiling --with-llvm, specify with CLANG=])
   fi

configure

@@ -4995,7 +4995,7 @@ done
 
 
   if test -z "$LLVM_CONFIG"; then
-  for ac_prog in llvm-config llvm-config-6.0 llvm-config-5.0 llvm-config-4.0 llvm-config-3.9
+  for ac_prog in llvm-config llvm-config-7 llvm-config-6.0 llvm-config-5.0 llvm-config-4.0 llvm-config-3.9
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2
@@ -5066,7 +5066,7 @@ fi
   # need clang to create some bitcode files
 
   if test -z "$CLANG"; then
-  for ac_prog in clang clang-6.0 clang-5.0 clang-4.0 clang-3.9
+  for ac_prog in clang clang-7 clang-6.0 clang-5.0 clang-4.0 clang-3.9
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2