[libclc] Convert README to Markdown (#111549)

A bit nicer to read on GitHub and with clickable links.

No content changes purely formatting.

Author: DavidSpickett

libclc/README.TXT renamed to libclc/README.md

@@ -1,20 +1,19 @@
-libclc
-------
+# libclc
 
 libclc is an open source implementation of the library
 requirements of the OpenCL C programming language, as specified by the
 OpenCL 1.1 Specification. The following sections of the specification
 impose library requirements:
 
-  * 6.1: Supported Data Types
-  * 6.2.3: Explicit Conversions
-  * 6.2.4.2: Reinterpreting Types Using as_type() and as_typen()
-  * 6.9: Preprocessor Directives and Macros
-  * 6.11: Built-in Functions
-  * 9.3: Double Precision Floating-Point
-  * 9.4: 64-bit Atomics
-  * 9.5: Writing to 3D image memory objects
-  * 9.6: Half Precision Floating-Point
+  * 6.1: Supported Data Types
+  * 6.2.3: Explicit Conversions
+  * 6.2.4.2: Reinterpreting Types Using as_type() and as_typen()
+  * 6.9: Preprocessor Directives and Macros
+  * 6.11: Built-in Functions
+  * 9.3: Double Precision Floating-Point
+  * 9.4: 64-bit Atomics
+  * 9.5: Writing to 3D image memory objects
+  * 9.6: Half Precision Floating-Point
 
 libclc is intended to be used with the Clang compiler's OpenCL frontend.
 
@@ -26,44 +25,43 @@ functions.
 libclc currently supports PTX, AMDGPU, SPIRV and CLSPV targets, but support for
 more targets is welcome.
 
-Compiling and installing
-------------------------
+## Compiling and installing
 
-(in the following instructions you can use make or ninja)
+(in the following instructions you can use `make` or `ninja`)
 
 For an in-tree build, Clang must also be built at the same time:
-
+```
 $ cmake <path-to>/llvm-project/llvm/CMakeLists.txt -DLLVM_ENABLE_PROJECTS="libclc;clang" \
     -DCMAKE_BUILD_TYPE=Release -G Ninja
 $ ninja
-
+```
 Then install:
-
+```
 $ ninja install
-
-Note you can use the DESTDIR Makefile variable to do staged installs.
-
+```
+Note you can use the `DESTDIR` Makefile variable to do staged installs.
+```
 $ DESTDIR=/path/for/staged/install ninja install
-
+```
 To build out of tree, or in other words, against an existing LLVM build or install:
-
+```
 $ cmake <path-to>/llvm-project/libclc/CMakeLists.txt -DCMAKE_BUILD_TYPE=Release \
   -G Ninja -DLLVM_DIR=$(<path-to>/llvm-config --cmakedir)
 $ ninja
-
+```
 Then install as before.
 
 In both cases this will include all supported targets. You can choose which
 targets are enabled by passing `-DLIBCLC_TARGETS_TO_BUILD` to CMake. The default
-is "all".
+is `all`.
 
 In both cases, the LLVM used must include the targets you want libclc support for
-(AMDGPU and NVPTX are enabled in LLVM by default). Apart from SPIRV where you do
-not need an LLVM target but you do need the llvm-spirv tool
-(https://github.com/KhronosGroup/SPIRV-LLVM-Translator) available. Either build
-this in-tree, or place it in the directory pointed to by `LLVM_TOOLS_BINARY_DIR`.
+(`AMDGPU` and `NVPTX` are enabled in LLVM by default). Apart from `SPIRV` where you do
+not need an LLVM target but you do need the
+[llvm-spirv tool](https://github.com/KhronosGroup/SPIRV-LLVM-Translator) available.
+Either build this in-tree, or place it in the directory pointed to by
+`LLVM_TOOLS_BINARY_DIR`.
 
-Website
--------
+## Website
 
 https://libclc.llvm.org/